1. Field of the Invention
The present invention relates to a radio transmitter used for communication methods using subcarriers such as the OFDM (orthogonal frequency division multiplex).
2. Prior Art
Generally, for modulated signals involving amplitude modulation, in particular, modulated signals involving multilevel modulation such as QAM (quadrature amplitude modulation), a linear operation is required of the radio-frequency power amplifier for transmitting power to the antenna. For this reason, as the operation class of the radio-frequency power amplifier, Class A and Class AB have been used.
However, with the migration to broadband communications, communication methods using subcarriers such as the OFDM have come to be used, and high efficiency cannot be expected of the conventional Class-A and Class-AB radio-frequency power amplifiers. That is, in the OFDM, high power is instantaneously generated in a completely random fashion by superimposing subcarriers on one another. That is, the ratio between the average power and the instantaneous peak power, PAPR (peak to average power ratio), is high. For this reason, it is necessary to always hold high DC power so that the instantaneous peak power can also be linearly amplified. In the Class-A operation, the power supply efficiency is only 50% at the most; in particular, in the case of the OFDM, the power supply efficiency is as low as approximately 10% since the PAPR is high.
Consequently, for example, in portable radios using batteries as the power source, the continuous usable time is short, which is inadequate for practical use.
To solve such a problem, a conventional EER (envelope elimination and restoration) method known as Khan's method has been proposed (see, for example, Patent Document 1).
FIG. 8 is a block diagram showing the EER method in outline. In FIG. 8, a modulated signal, for example a QAM signal, outputted from an OFDM signal generation circuit 100 as modulated signal generating means is divided into two branches. On one branch, the modulated signal is upconverted by a quadrature modulation circuit 111, and inputted as a radio-frequency modulated wave to a radio-frequency modulated signal input terminal of a radio-frequency power amplifier 112 comprising a saturated amplifier. On the other branch, the modulated signal is converted to an amplitude component by an amplitude extraction circuit 101, amplified to a desired amplitude component voltage level by an operational amplifier 103, and inputted to an emitter follower 106 as voltage converting means. The emitter follower 106 supplies the current required by the radio-frequency power amplifier 112 to the supply voltage terminal of the radio-frequency power amplifier 112 together with the amplitude component. In FIG. 8, reference numeral 102 represents an amplitude amplification circuit as amplitude amplifying means, reference numeral 104 represent a feedback circuit, and reference numeral 105 represents a supply voltage portion.
When a radio-frequency modulated wave which is an input signal is amplified by the radio-frequency power amplifier 112 comprising a saturated amplifier, the amplitude component of the radio-frequency modulated wave is temporarily lost in the process. However, when the supply voltage of the radio-frequency power amplifier 112 is controlled in proportion to the amplitude component of the radio-frequency modulated wave which is the input signal, the amplitude component of the radio-frequency modulated wave is inputted from the supply voltage portion 105 to the supply voltage terminal of the radio-frequency power amplifier 112. As a result, a radio-frequency modulated wave containing the original amplitude component which radio-frequency modulated wave is restored at the radio-frequency power amplifier 112 is outputted.
The provision of this structure enables the radio-frequency power amplifier to operate in a highly efficient saturated condition even when the amplitude component of the modulated signal is changed, so that the efficiency of the radio-frequency power amplifier 112 can be enhanced.
Here, when the radio-frequency power amplifier comprises, for example, a field-effect transistor, the saturated amplifier refers to a Class-F amplifier where harmonic control is performed so that the drain voltage waveform is rectangular, or a Class-E or a Class-D amplifier where the load condition is optimized so that the drain voltage waveform and the drain current waveform do not overlap each other. In this saturated amplifier, since the period during which the drain current and the drain voltage are simultaneously generated is minimized as a characteristic thereof, power consumption can be suppressed.
For example, when a supply current and a supply voltage of 200 mA and 3V are supplied, the DC power is 600 mW. However, in the saturated amplifier as the radio-frequency power amplifier 112, no current flows when the field-effect transistor is off, and only the supply voltage is applied. Consequently, the DC power consumption is 0. On the other hand, although a current of 200 mA flows when the field-effect transistor is on, since the field-effect transistor is completely conducting, it can be assumed that the drain-source voltage VDS is a saturated voltage, that is, 0.3 V at the most. In this case, a DC power of 0.3 V×0.2 A=0.06 W, that is, 60 mW is consumed in the field-effect transistor. The power supply efficiency reaches as high as (600−60)/600=90%. This effect is significant because the power supply efficiency reaches only 50% at the most in the Class-A amplifier.
Generally, in transmitters using the EER method, unless the amplitude component and the phase component obtained from the output terminal of the radio-frequency power amplifier are precise reproductions of the phase component and the amplitude component of the original modulated signal, the original modulated signal (precisely, radio-frequency modulated wave since the original modulated signal is frequency-converted) cannot be reproduced. That is, the errors of the amplitude component and the phase component appear as a spectral distortion of the outputted radio-frequency modulated wave and degradation in modulation accuracy.
For this reason, in the EER method, it is necessary to output, from the modulated signal generator 100, a modulated signal having undergone an inverse correction processing to previously obtain the error function of the amplitude component and the phase component and multiply the amplitude component and the phase component by the inverse function of the error function.
Moreover, it is necessary for the transmission path of the amplitude component to have a closed loop structure having a feedback circuit so that the error component reaching the supply voltage terminal of the radio-frequency power amplifier is reduced.
When the saturated amplifier is used in a completely saturated region, the amount of fluctuations in the phase change with respect to the change in the amplitude component inputted to the supply voltage terminal of the radio-frequency power amplifier is increased, so that the arithmetic throughput of the inverse correction processing is increased. In addition, for example, when the characteristic of the radio-frequency power amplifier is changed due to fluctuations in temperature or power, since the amount of fluctuations in the error function is increased, a spectral distortion or degradation in modulation accuracy occurs in the radio-frequency modulated wave as a result.
For this reason, as the saturated amplifier characteristic to be used, a region that is not completely saturated, that is, a region close to the saturated region, for example, the 1-dB gain compression point is used.
Patent Document 1: U.S. Pat. No. 5,251,330A1
However, as the modulated signal, in a modulated signal with a high PAPR and high instantaneous peak power like that of the IEEE 802.11a/g standard which is a wireless LAN standard, there are cases where by the transmitter characteristic fluctuating, a time delay occurs between a first amplitude component outputted from the amplitude extraction circuit 101 and a second amplitude component of a radio-frequency modulated wave inputted to the radio-frequency modulated signal input terminal of the radio-frequency power amplifier 112, amplified and occurring at the output terminal of the radio-frequency power amplifier 112.
In this case, the following problem arises: When the occurring time delay is the allowable level of the inverse function of the amplitude component and the phase component, the distortion of the radio-frequency modulated wave never occurs. However, when the second amplitude component leaks to the feedback circuit 104 as shown in FIG. 8, the leakage component and the first amplitude component are superimposed on each other in the feedback circuit 104. This causes ringing due to a transient response, so that the amplitude component outputted from the amplitude amplification circuit 102 constituting the closed loop oscillates without converging. Consequently, the modulated waveform cannot be reproduced and a distortion occurs at the output terminal of the radio-frequency power amplifier 112, that is, the saturated amplifier.
Generally, as the modulated signal, in a modulated wave where the PAPR is high and the difference between the average power and the instantaneous peak power is large like that of the IEEE 802.11a/g standard which is a wireless LAN standard, in a linear amplifier where no amplitude component is required of the supply voltage of the radio-frequency power amplifier, a large capacitance can be set in the vicinity of the supply voltage terminal in order to suppress the modulated wave characteristic degradation due to supply voltage fluctuations.
However, in the EER method, it is necessary for the supply voltage terminal of the radio-frequency power amplifier to linearly transmit an amplitude component having a frequency component of, for example, approximately 40 MHz. For this reason, a capacitance having a high capacitance value cannot be set at the supply voltage terminal of the radio-frequency power amplifier. That is, the capacitance value of the capacitance stays at a value where at least a signal of 40 MHz can pass. Therefore, the error of the amplitude component is inputted to the supply voltage terminal of the radio-frequency power amplifier 112 without attenuated. In that case, a linear distortion such as a ripple occurs. This appears as a spectral distortion of the modulated wave and degradation in modulation accuracy as a result.